As semiconductor device geometries continue to scale down below 0.25 .mu.m, and approach 0.13 .mu.m minimum feature size, the metal interconnect lines which carry current between devices on a chip begin to dominate the overall circuit speed. In order to enhance interconnect speed and reliability, the semiconductor industry is moving away from blanket deposition and etch of aluminum (AL) based metallizations towards single-damascene and dual-damascene interconnect structures with copper (Cu) based metallizations. Copper is a lower resistivity metal than aluminum, which results in lower RC interconnect delay. Copper has also been shown to have superior electromigration performance over aluminum, but is more difficult to process, primarily because (1) it is more difficult to etch and (2) it acts as a deep level trap in silicon (Si) based devices. The preferred way to process copper interconnects is to (1) etch a trench or via into a dielectric material, (2) deposit the interconnect metallization to fill the trench or via, and then (3) polish the metal back to remove any metal from the field (surface of the wafer). The resulting metal-filled trenches and vias form the electrical interconnect. Forming an interconnect structure by filling a trench or via with metal is known as a damascene process. If a trench and underlying via are filled simultaneously, it is known as a dual-damascene process.
Damascene processes are discussed in "Tantalum, Copper and Damascene: The Future of Interconnects" by Peter Singer, in Semiconductor International, June 1998, and "Processing And Integration of Copper Interconnects " by Robert L. Jackson, Eliot Broadbent, Theodore Cacouris, Alain Harrus, Maximillian Biberger, Evan Patton, and Tom Walsh.
A refractory metal (such as Ti, TiN, Ta, TaN, or WN) is typically deposited prior to the deposition of aluminum or copper based metallizations in damascene processing. This barrier layer prevents copper diffusion into the surrounding dielectric and improves the quality of the metal/dielectric interface.
One of the most important reliability concerns in integrated circuits today is electromigration in the metal interconnects, leading to open circuits or short circuits. Under an applied current, charge carriers in a metal (generally electrons) can impart momentum to the metal atoms in an interconnect, inducing an atomic flux in the direction of the charge carriers. In regions where there is a divergence in the atomic flux, atoms can be depleted to form voids and eventually an open circuit, or atoms can be accumulated to form metal extrusions leading to a short circuit with an adjacent interconnect. Electromigration flux divergences in an interconnect line typically occur (1) at the line end, where a via of a different metal blocks atomic diffusion; (2) due to microstructure variations, which give rise to variations in the effective atomic diffusivity along the interconnect; or (3) due to other geometric or microstructural variations such as blocking precipitates across the line width or fluctuations in the line profile. Copper interconnects theoretically have higher electromigration resistance than aluminum at operating temperatures. This improved electromigration reliability in copper has been observed in controlled experiments, but has been difficult to produce in a production environment.
Three process issues that affect electromigration reliability for copper interconnects, manufactured in a production environment, are listed below:
(1) Damascene structures typically have small metal grains compared to etched metal lines. The small metal grains are a result of the metal being deposited into a constrained via and trench, as opposed to being deposited as a blanket film in the case of etched metal lines. Interconnects with large grains typically show better electromigration reliability because they provide fewer grain boundary diffusion paths during electromigration. Atoms diffuse faster along grain boundaries than through the bulk of the grains. PA1 (2) Damascene interconnects often contain seams that form during the filling process. These seams can act as a fast diffusion path during electromigration. PA1 (3) Copper interconnects tend to form a weaker interface with the barrier and passivation materials than aluminum interconnects. As a result, the interface of a copper interconnect line with the surrounding barrier and dielectric materials can act as a fast diffusion path for electromigration. Aluminum interconnects typically react with the surrounding materials to form a strong, low diffusivity interface.
Copper interconnects manufactured in a production environment typically show electromigration reliability that is much lower than predicted theoretically. In order to effectively use copper interconnects in high reliability devices, a technique must be found to (1) increase the mean time to failure (MTTF), and (2) reduce the standard deviation in the mean time to failure (.sigma.) during electromigration testing.